The present invention relates to inductance measuring apparatus, and more particularly to proximity sensing systems which utilize a variable inductance measuring apparatus to determine the inductance of a sensor inductor as a means of monitoring the proximity of a target object to a sensor unit.
Proximity switches, and the advantages thereof, as well known in the art. In many applications of switching electric current to a load, there are situations which require the positioning of a switch assembly in environmental conditions which render the reliable life of the switch assembly short and subject to uncertain operation. One example of an electric switch assembly which must operate under adverse conditions is presented by the aviation industry. On an airplane, numerous switch assemblies are subjected to extreme heat and extreme cold, high vibration and shock, as well as to chemicals, corrosive fluids and adverse moisture conditions. Conventional mechanical switches simply are not reliable under the extreme environmental conditions encountered in aircraft operation; and yet, in many applications on an aircraft it is essential to have a current control and switch assembly with remote operational capability which provides assured, reliable operation.
In such situations, proximity-type switches have proved to be able to reliably withstand extreme environmental conditions. Such a proximity-type switch typically comprises a so-called two-piece proximity sensing system which includes a sensor unit, including an inductor having a coil and a core, located at a remote site, and also includes electronic switching circuitry interconnected with the sensor unit by a cable and responsive to changes in the inductance of the sensor inductor. As is well known in the art, the inductance of the sensor inductor varies in accordance with variations of the reluctance of the magnetic path which includes the sensor inductor and a target object (e.g., part of a landing gear strut or a cargo door). By determining the inductance of the sensor inductor, the electronic switching circuitry can provide an indication of proximity of the target object to the sensor unit. It is also well known in the art that the sensor unit can be made physically robust in order to withstand the extremes of environment encountered in operation, with the more delicate electronic switching circuitry of the proximity sensing system being isolated from the environmental extremes at the site of the sensor unit.
The above notwithstanding, environmental and other conditions can still affect the accuracy of the inductance measurement, and consequently the reliability of the switching of the proximity sensing system. This is because the sensor unit is actually characterized by an equivalent circuit which includes, in addition to the inductance of the sensor inductor, an equivalent series resistance corresponding to the resistance of the inductor coil and the cable and an equivalent shunt capacitance due to the cable. Through the proper choice of core materials, the sensor unit can be fabricated so that its equivalent inductance is relatively temperature independent. The equivalent series resistance and shunt capacitance of the sensor unit, however, are temperature dependent causing an uncompensated inductance measurement to also be variable with temperature.
Also, it is desirable that the electronic switching circuits and sensor units of different proximity sensing systems be interchangeable, that is, any one electronic switching circuit can operate and maintain calibration when connected to any sensor unit and its associated cable of an arbitrary length. As a result of this interchangeability, the resistance in series with the sensor inductor and particularly, the capacitance in shunt with the sensor inductor can vary over a substantial range. Thus, it is desirable that no special adjustment or recalibration of a proximity sensing system be necessary when its electronic switching circuitry is interconnected with various sensor unit and cable assemblies.
In order to reduce the temperature dependence of two-piece proximity sensing systems, prior art devices of this type often incorporate a temperature compensation network into the sensor unit. An example of a temperature compensated sensor unit for a two-piece proximity switch appears in U.S. Pat. No. 3,454,869. Therein is disclosed a proximity sensing system which includes a Maxwell bridge, including an inductive leg and a plurality of resistive legs, and a synchronous detector which is responsive to changes in the inductance of the inductive leg of the bridge. The inductive leg and one of the resistive legs comprise the sensor unit. The resistive leg in the sensor unit is chosen to provide a degree of temperature compensation so as to reduce the temperature variability of the inductance measured by the proximity sensing system. While such a sensor unit provides a degree of temperature invariability, certain factors affecting the accuracy, as well as the reliability, of this type of proximity sensing system remain.
First of all, the incorporation into the sensor unit of a temperature compensation resistor does not insure the effective cancellation of temperature-dependent effects from the equivalent series resistance of the sensor unit over the wide temperature variations encountered in many applications (for instance, in the aviation industry). Also, since the temperature compensation resistor generally incorporates a wire-wound configuration utilizing relatively fine wire, it introduces a failure point which affects the reliability of the sensor unit. In addition, the use of a temperature compensation resistor requires three leads to the sensor unit, increasing the amount of cabling required to connect the remote sensor unit to the electronic switching circuitry of the proximity sensing system (an important consideration in areas, such as the aviation industry, where the cost, maintainability and weight of wiring are factors). Further, the three-wire requirement for sensor units incorporating a temperature compensation resistor introduces another failure point, an additional reliability factor which must be considered. And, of course, the use of a temperature compensation resistor incorporated into the sensor unit does not affect the temperature variability arising from the equivalent shunt capacitance of the sensor unit.
It is therefore a general object of the present invention to provide an improved inductance measuring technique which can be adapted for use in a proximity sensing system.
A further object of the present invention is to provide a proximity sensing system incorporating a variable inductance measuring apparatus capable of providing an accurate, temperature independent measure of the inductance of a sensor inductor despite the presence of equivalent series resistance and shunt capacitance which is variable with changes in temperature and in the length of the cable coupling the sensor inductor and the variable inductance measuring apparatus.
Another object of the present invention is to provide such a proximity sensing system incorporating a remote sensor unit comprising only a single circuit element, an inductor requiring only two leads, one of which is common or ground, thereby reducing wiring and connection requirements with attendant increases in economy, maintainability and reliability.
Still another object of the present invention is to provide a proximity sensing system incorporating a variable inductance measuring apparatus capable of measuring a variable inductance and thereby developing a target proximity indication with increased accuracy and reliability over devices currently available, and at a lower cost.